1. Field of the Invention
This invention relates to sputtering and more particularly to etching during sputtering in the presence of negative ions.
2. Brief Description of the Prior Art
The formation of negative ions in the sputtering of highly ionic compounds such as TbF.sub.3 was reported by Hanak et al "Effect of Secondary Electrons and Negative Ions on Sputtering of Films", J. Vac. Sci. Technol., Vol. 13, No. 1, Jan./Feb. 1976, pp. 406-409. It describes cosputtering from a target of TbF.sub.3 and a target of ZnS, with etching of the substrate directly opposite the TbF.sub.3 target. The article states "It was postulated that etching of the substrate over the TbF.sub.3 was caused by the sputtering of the substrate by F.sup. ions first released by the target and then accelerated and focused onto the substrate at full target-substrate voltage. . . " The Article also speculates that other negative ions "such as those observed by Honig, [Refs.] 3,4 would be expected to produce a similar effect," referring to oxides, and ZnS, involving sulfide and oxide ion bombardment. Both examples selected were those of non-metals. Halide ions are also mentioned. Honig is cited by Hanak as referring to Cl.sup.-, F.sup.-, Br.sup.-, O.sup.-, OH.sup.-, O.sub.2.sup.-, S.sup.-, C.sub.1.sup.- to C.sub.10.sup.-, Sh.sup.-, Ag.sub.2.sup.- and Ag.sub.2 O.sup.- as sources of negative ions. No mention of any intermetallic compounds or any rare earth (RE) metallic compounds is made or in any way suggested. Ag.sub.2.sup. - was of relatively small intensity, involving few ions as compared to Ag.sub.1.sup.+, Ag.sub.2.sup.+ and Ag.sub.2.sup. + and Ag.sub.3.sup.+. Thus, little etching would be provided, if any, as our discoveries show silver to be a poor source of negative ions for etching.
Applicants present experimental data herein which shows that negative ion formation has a significant effect on the sputtering of certain intermetallic compounds and not just the highly ionic compounds previously reported by Hanak et al. In addition, applicants present a model based on electron affinity data which is useful in predicting which compounds and intermetallics will have a high negative ion
Middleton et al "A Close to Universal Negative Ion Source," in Nuclear Instruments and Methods 118, (1974) 329-336, shows how to generate negative ion currents of metals such as Li, Ca, Ti, Cu and Ni by directing a beam of cesium positive ions from a surface ionization source to sputter a hollow target cone to generate negative ions. No suggestion is made as to an application for which such ions might be used.
U.S. Pat. No. 3,573,454 of Andersen, entitled "Method and Apparatus for Ion Bombardment Using Negative Ions" teaches using bombardment of an alumina (AL.sub.2 O.sub.3) specimen with negative O.sub.1 ions to produce sputtered Al.sup.+ ions. Such a process can be used for ion implantation for manufacturing solid state electronic devices. a multicompanent finely divided
U.S. Pat. No. 3,855,612 of Rosvold, entitled "Schottky Barrier Diode Semiconductor Structure and Method" teaches sputtering from a target formed of two materials required to form a nickel, platinum, silicon ternary compound of platinel silicide. The target is a disk of nickel with strips of platinum formed on it. The substrate is silicon. That procedure simply used a multicompnent target without suggesting that a target should include an alloy or compound of the two metals or at the very least extremely finely divied particles. For the purpose to be achieved in this invention, our experiments havve shown tht placing a strip of sumarium upon a gold target will not yield any measurable etching by negative ions of gold.
The importance of negative ion formation in sputtering came to our attention while studying the deposition of rare earth - gold alloys. We observed that under conditions that produced a good accumulation rate of GdAu we obtained zero accumulation for a SmAu target. Closer examination of the substrates used in the SmAu deposition experiments revealed that we were getting, in fact, a negative accumulation rate, i.e., etching of the substrates. The etching occurred in a sharply defined region which closely matched the size and shape of the ground shield opening of the target. Furthermore, the etching phenomona did not appear to be chemical in nature because different substrates, e.g. Al.sub.2 O.sub.3, and Mo, all etched and at comparable rates.
We hypothesized from these observations that: 1) the etching was caused by sputtering; 2 the ions causing the sputter etching were being accelerated away from the cathode and therefore must be negative; and 3) the ions probably had a high mass. After a series of sputter etching and Secondary Ian Mass Spectrometry (SIMS) experiments we have concluded that certain target materials have a surprisingly large yield of negative ions which can significantly effect the accumulation rate, composition and properties of sputtered films. SIMS is a process in which a beam of positive argon ions is directed upon a sample and ejected ions are analyzed by a mass spectrometer. While this phonomenon has been reported for highly ionic target materials such as TbF.sub.3 by Hanak et al, it appears to be much more generic in scope than previously speculated and several additional species of compounds including intermetallic compounds not suggested by the prior art have been found to yield even stronger etching than TbF.sub.3. Nothing in the prior are suggested that significant quantities of negative metallic ions of high energy could be generated in a sputtering chamber.
In accordance with this invention, a negative-ion-source-material comprises a source of negative ions of a metal B included in an intermetallic composition including a metal A and a metal B. Metal B is a metal which when combined with metal A is adapted to be ionized to a substantial degree to form highly energetic negative ions of metal B upon bombardment by energetic particles of atomic dimensions.
Preferably, the metal A has an electronegativity and metal B has an electronegativity X.sub.B such that X.sub.B - X.sub.A is greater than about 2.55 electron Volts. Where metal B is gold, the metal A has an electronegativity less than about 3.22 electron Volts.
In any event, it is preferred that the negative ions produced should have sufficient energy to produce significant etching.
Further, in accordance with this invention, at target voltages below 3,000 Volts, at appropriate electronegativity differences greater than about 2.55 electron Volts, the target is composed of a composition selected from groups in which the B metal is Au, and the A metal is selected from the group including: Li, Na, K, Rb, Si, In, Cs, Ba, La, Ce, Pr, Nd, Pn, Sn, Eu, Tb, and Dy; in which the B metal is Pt and the A metal is selected from the group including: Li, Na, K, Rb, Cs, Ba, La, Ce, Pr, and Nd; in which the B metal is selected from the group including Ir, Hg, Po, and Os and the A metal is selected from the group including: K, Rb, Cs, and Ba; in which the B metal is Os and the A metal is selected from the group including Rb and Cs, and in which the A metal is Cs and the B metal is selected from the group including Sb and Zn.
It is to be understood that the compositions can include alloys, and other mixtures such as compressed well mixed powders, insofar as the elements are extremely finely mixed with homogeneity. Other elements can of course be present as will be obvious to those skilled in the art to the extent that they do not tend to inhibit the formation of negative ions.
In a preferred embodiment the source material is selected from a group of gold intermetallic compositions with an element selected from the group consisting of Ba, Ce, Pm, Sm, La, Eu, Pr, Nd, La, and Cs.